A typical single mode optical fiber has a refractive index distribution as shown in FIG. 1. Hitherto, such a refractive index distribution was mainly achieved by the addition of an additive which increases the refractive index of the glass in a core of the optical fiber. An additive, such as an oxide GeO.sub.2, P.sub.2 O.sub.5 and Al.sub.2 O.sub.3 is usually used. However, such an additive may cause some problems such that (1) attenuation of light transmission of the optical fiber is increased by an increase of Rayleigh scattering, (2) bubbles or crystal clusters are induced by the additive in the glass preform and (3) the glass preform tends to crack due to an increase of the coefficient of thermal expansion of the glass. Therefore, the lower the content of the additive in the glass preform, the better.
For this reason, it is proposed to increase the refractive index difference between the core and cladding by the addition of an additive which lowers the refractive index of glass in the cladding. Examples of such additives are B.sub.2 O.sub.3 and fluorine and their combination. B.sub.2 O.sub.3, however, has disadvantages in that it increases the coefficient of thermal expansion of the silica glass and in that it has an absorption loss in a longer wavelength region. Thus, fluorine is preferably used as the refractive index-lowering additive.
The VAD method or the OVPO method in which a porous soot preform is produced by flame hydrolysis of glass raw materials is known as an economical and highly productive method for producing an optical fiber. It is, however, very difficult to add fluorine in a sufficient amount to lower the refractive index of the cladding by such a method utilizing flame hydrolysis. For example, Japanese patent publication No. 15682/1980 discloses a method for adding fluorine to a glass preform, by which the refractive index is lowered by only 0.2 to 0.3%. This means that the amount of fluorine to be added has a limit in this method.
Japanese patent Kokai publication (unexamined) No. 67533/1980 discloses a method for effectively adding fluorine to the glass preform by heating a deposit of fine glass particles in an atmosphere containing a fluorine-containing compound. It is, however, difficult to use this method to adequately distribute fluorine in the glass preform and thus to achieve, by the sole use of fluorine, the refractive index distribution shown in FIG. 1, which is essential to produce a practically operative optical fiber.
A method schematically illustrated in FIG. 2 is proposed as a productive method for producing an optical fiber containing fluorine and having a practically operative refractive index distribution by utilizing formation of the fine glass particle deposit by flame hydrolysis.
In FIG. 2, while rotating and gradually lifting up a glass rod 2 which constitutes the core and attached to a lift-up device 1, fine glass particles which are produced by means of a burner 3 are deposited on the surface of the glass rod 2 to form a porous glass layer 4 corresponding to the cladding. The fine glass particles are produced by supplying the burner 3 with hydrogen, oxygen and glass raw materials such as SiCl.sub.4 simultaneously and flame hydrolyzing them. In FIG. 2, numerals 5 and 6 represent a reactor and an outlet, respectively. The thus formed composite of the glass rod and the porous glass layer is heated in an atmosphere containing fluorine in order to add fluorine to the porous glass layer and simultaneously to make the glass layer transparent in order to form a transparent glass preform having a refractive index distribution shown in FIG. 1. If the thickness of the cladding is not sufficient at this stage, the transparent glass preform is drawn and the fine glass particles are again deposited on the surface of the drawn glass preform and heated in an atmosphere containing fluorine. This procedure may be repeated to obtain the cladding having a desired thickness.
In the above-described method utilizing the apparatus of FIG. 2, the glass rod which constitutes the core is often produced by heating and drawing the rod to a desired diameter in an atmosphere containing water vapor. This may result in contamination of the glass rod surface with hydroxyl groups. Particularly, when the glass rod is drawn in a flame formed by burning combustion gas containing hydrogen, the glass rod surface is severely contaminated with the hydrogen groups. In addition, during the formation of the porous glass layer corresponding to the cladding, the glass rod surface tends to be contaminated with the hydroxyl groups derived from water vapor generated in the flame for synthesizing the fine glass particles.
When the transparent glass preform, having the surface of the core contaminated with the hydroxyl groups, is drawn to form an optical fiber, light propagated through the optical fiber suffers from absorption loss due to the hydroxyl groups and hence, the light transmission characteristics of the optical fiber are deteriorated. When the optical fiber is used as a single mode optical fiber, the light transmission is particularly affected by the presence of an interface layer contaminated with the hydroxyl groups between the core and cladding, and the transmission characteristics are remarkably deteriorated since power distribution in the single mode optical fiber reaches the cladding.
For example, a pure quartz rod with very low hydroxyl group content (up to about 10 ppb) is drawn to a diameter of 12 mm in an oxyhydrogen flame. Thereafer, by means of the apparatus of FIG. 2, on the surface of the drawn quartz rod, a porous glass layer of pure silica glass is formed. The outer diameter of the glass layer is 110 mm. The thus produced composite of the quartz rod and the porous glass layer is heated in an atmosphere containing fluorine to obtain a transparent glass preform of 45 mm in outer diameter having a refractive index distribution as shown in FIG. 3.
Then, the glass preform is drawn in the oxyhydrogen flame to a diameter of 12 mm, on which a porous glass layer is again formed by means of the apparatus of FIG. 2. The outer diameter of the glass layer is 110 mm. The thus produced composite is heated in an atmosphere containing fluorine to obtain a transparent glass preform having a refractive index distribution as shown in FIG. 4.
The glass preform is drawn to a predetermined diameter, inserted in and integrated with a commercially available quartz tube, and then drawn to form an optical fiber, resulting in single mode operation at a wavelength of 1.3 micrometer. Its attenuation of light transmission at a wavelength of 1.3 micrometer is 4.0 dB/km and that at wavelengths of 1.39 micrometer due to the presence of the hydroxyl groups is 150 dB/km. These results mean that the hydroxyl groups are formed in the drawing step in the oxyhydrogen flame.